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Concrete

On this page go to
Subgrade
The concrete mix
Reinforcement
Placing the lining
Concrete thickness
Finishing and curing
Joining
Performance
Cost
Failures
Repair
Related pages

Pages in this section include:

  Hard surface lining techniques
Concrete
  Shotcrete
  Grouted fabric mats
  Soil-cement lining
  Flumes and pipes
  Tiles and bricks
  Asphalt

Concrete linings are widely used, with benefits justifying their relatively high cost. They are tough, durable, relatively impermeable and hydraulically efficient. Concrete linings are suitable for both small and large channels and both high and low flow velocities. They fulfil every purpose of lining.

Properly designed, constructed and maintained concrete linings should have an average service life of over 40 years (USBR, 1975; Kraatz, 1977; Stevenson, 1999). If the deteriorating action of salts and the development of cracks can be checked or do not occur, the linings can last indefinitely. They are usually subject to some cracking caused by collapsible soils, expansive clays, freeze-thaw action and frost heave, but cracks that permit appreciable leakage can be sealed with asphaltic compounds. Costly maintenance is seldom necessary provided that the installation was performed correctly and the subgrade is not allowed to deteriorate to the extent that extensive cracking, and eventually concrete failure occurs. Maintenance of a deteriorating concrete lining is very expensive.

A successful concrete lining must be durable and remain substantially watertight, to give many years of low-maintenance service life. The success of a concrete liner is highly dependent on its installation and the materials used.

Figure 1 Success factors for concrete lining
Source: Developed from USBR, 1988.
Source: Developed from USBR, 1988.

Subgrade Top button

Concrete linings should be placed on well-consolidated subgrade. Preparation should include filling all voids with suitable material, ensuring adequate compaction of the rest of the foundation by rolling, tamping or vibrating, and trimming the foundation to the correct shape (Stevenson, 1999).
Subgrade preparation should be performed far enough in advance to avoid delay of the lining operations. At the time of placement of the concrete the subgrade should be adequately moist. This is achieved by sprinkling water in a manner that does not form mud or puddles.
In addition it may be important to consider some of the general issues in relation to the channel subgrade and prism that are discussed in Hard surface lining techniques.

The concrete mix Top button

Concrete used in channel linings should be mixed so that it is plastic enough to consolidate well and be stiff enough to stay in place on the slopes (USBR, 1988).

For hand placing and for placing with lighter machines the consistency should be such that the concrete will barely stay on the slope. A slump of 50-65mm is usually satisfactory. For heavier longitudinally operating slip-form machines, best results are obtained with a slump of about 50mm (USBR, 1988).
A close control of consistency and workability is important, as a variation of 25mm in slump can upset the established operating adjustments and interfere with progress and quality of work. When placing the concrete lining with a subgrade-guided slip form, the slump has a critical effect on slip-form operation because if the concrete is not sufficiently plastic it is difficult to control thickness of the lining (USBR, 1988).

Concrete aggregate should be clean, hard and durable. The concrete aggregate for channel lining should include enough well-graded sand to ensure a reasonably good finish with the minimum treatment specified. Use of more sand than necessary for this purpose should be avoided. Entrainment of from 3.5-5.5% air also helps materially in securing a satisfactory finish. Another factor that will considerably improve finishability of the concrete is the reduction of pea gravel (5-10mm) content of the mix to about 5%. USBR specifications for channel lining usually stipulate this separation where sufficient quantities are required (USBR, 1988).

Proper consolidation is chiefly dependent on the mix, consistency and placing procedure (USBR, 1988).
The use of hydraulic lime has been tested in combination with Portland cement with the aim of developing a lining with less shrinkage and able to withstand greater tensile deformation without cracking. These tests have been successful (USBR, 1988). Other special cements may be required when certain soluble sulphates are present in the soil in appreciable quantities.

Reinforcement Top button

Most concrete linings installed in older channels in the United States were reinforced. During recent years reinforcement has been omitted wherever possible to reduce construction costs and because it does not materially improve effectiveness or durability if transverse joints are provided at sufficiently frequent intervals to control intermediate cracking (USBR, 1988). In Australia, older channels were generally 50-75mm thick and rarely reinforced (Stevenson, 1999). Following problems experienced with these concrete channels it has been tempting to include reinforcement in some new channels.
However, it is noted in the literature that it is generally better to use unreinforced concrete (Stevenson, 1999). Reinforcement cannot be justified, except under unusual conditions such as high back pressure, high flow velocities in the channel, where movement of the subgrade is a possibility, or in reaches where failure would endanger life and property adjacent to the channel. Unreinforced concrete linings are more susceptible to damage by hydrostatic pressures, subgrade movement or failure under the linings than reinforced concrete, but not to the degree that the difference in cost might suggest. Unreinforced concrete fractures more readily than reinforced concrete, thus relieving the pressure and reducing the area of damage (Swihart and Rutenbeck, 2001).

Steel reinforcement does not prevent cracking. It should, however, control crack width and hold the pieces of a badly cracked section of the channel together. Unfortunately, steel reinforcement complicates construction and poses the risk of deterioration due to corrosion of the steel. On balance it is better to use unreinforced channel linings, except possibly in areas of turbulence, where the maintenance of integrity despite cracking could prevent loose pieces of lining being lost (Stevenson, 1999).

When the lining is reinforced, consolidation of the concrete is both difficult and uncertain unless the steel is held firmly in its proper position in the middle of the slab and not permitted to sag during placing operations. This is not easily accomplished and examinations reveal that steel is often much lower than it should be. When it sags during concrete placing, there is poor consolidation under the steel. As a consequence steel is often exposed and corrodes.

To ensure proper positioning and prevent displacement, reinforcement must be adequately tied and supported. Rocks or precast concrete blocks are commonly used satisfactorily as supports if adequate in size and spaced at proper intervals. When a general downward displacement of the reinforcement cannot be entirely avoided, an allowance should be made in setting the steel to compensate for the displacement.

A more recent option for reinforcement of concrete involves the use of fibre reinforcement, where either synthetic of steel fibres (approximately 20-40mm long) are mixed with the concrete prior to construction of the lining. Recent case studies in the United States have experimented with this form of reinforcement in shotcrete (USBR, 1994).

Placing the lining Top button

Placing methods for concrete range from the hand method commonly used on small channels or laterals to the longitudinally operating slip-form machine designed for lining of large channels.

Manually placed concrete liners

Placing concrete liners by hand may prove economical when low-cost labour is available or when reach of channel is too short or its cross-section too small to be economical for mechanised placement. Success relies on good formwork or guides, the right concrete for the job, and sufficient labour to handle the work (Stevenson, 1999).

The simplest hand operation is placing unreinforced lining in small laterals and farm ditches where the concrete is dumped and spread on the sides and bottom in alternate sections. Screed guides are laid on the subgrade and the concrete is screed up the slope to proper thickness. Guide rails or boards must be set up carefully and fixed securely in place, as the line and level of the whole channel depends on them. The channel shape profiles suspended from the guides and used as screed rails must also be accurate and robust enough to be used repeatedly without distortion or collapse (Stevenson, 1999). Three-metre screed planes are practicable for two-person operation. Manually placed concrete liners are consolidated mainly in the screeding operation. One or two passes with a long-handled steel trowel completes the finishing. Transverse grooves are cut at 2m intervals, and the lining is cured by use of sealing compound. Mixes for this method should be well sanded to simplify placing and finishing. (USBR, 1988).

Larger linings constructed by hand are usually placed in alternate panels to facilitate placing, finishing and curing operations. There may also be some reduction in overall shrinkage cracking if enough time elapses before placing the intervening panels. In this method, it is best to place the bottom slab first to provide support at the toes of the side panels. The panels are screed up the slope, and the concrete is vibrated ahead of the screed (USBR, 1988).

If forming large rectangular channels in suitable (non-expansive) soils, the sides can be made vertical and poured against conventional formwork. In this case the concrete should be at least 100mm thick, partly for robustness and partly so that it can be compacted with an immersion vibrator (Stevenson, 1999).

Slip-form concrete liners

More efficient placement of concrete on slopes is accomplished by use of weighted slip forms, similar to the weighted slip form depicted in the figure below. After excavation and trimming of the subgrade, pouring, shaping, compacting and smoothing of the concrete lining are done with the slip form. Slip forms are used for all sizes of channel.

Figure 2 Slip-form screed for placing concrete on side slopes
Figure 2 Slip-form screed for placing concrete on side slopes
Source: USBR, 1976.

The screed may be pulled up the slope by equipment on the berm or by air hoists mounted on the slip form. Concrete should be vibrated internally just ahead of the slip form. Under proper conditions of operation the surface made by the slip form requires no further screeding and very little finishing. The slip form itself should not be vibrated, as this procedure causes a swell in the concrete at the lower edge. This excess concrete is not only laborious to remove but it also emphasises sags that tend to form at longitudinal bars.

For many years, concrete channel linings have been built in the United States with longitudinally operated lining slip forms. This type of channel lining is only cost-effective where there are long stretches of channel with uniform cross-section that require lining. It is for this reason that longitudinal slipforms are rare in Australia.

The longitudinal slip form is a steel plate, curved at the leading edge, extending across the bottom and up the slopes of the channel and shaped to conform to the finished surface of the lining. A distributor plate when used is fastened to the leading edge of the slip form and extends upward on a steep incline to the working platform.

On some machines a continuous row of hoppers in the working platform feeds into drop chutes, each supplying one compartment of the trough below. As the concrete is distributed through the bottom of the trough and under the slip form, a vibrating tube parallel to and a few centimetres ahead of the leading edge of the form consolidates the concrete. The trailing edge of the slip form is usually adjustable to positions somewhat lower than that of the leading edge. This provision improves consolidation and moulds the concrete more closely to the subgrade.

Modern longitudinal slip forms commonly used in the United States today on large channel lining operations can be described as travelling channel-lining machines. These machines frequently exceed their design rate of lining of approximately 500m/day. Many improvements have been made in the longitudinally operating slip-form machines for lining channels of all sizes. Large hydraulically operated lining machines have been developed, with some electrically controlled to line and grade. Preformed longitudinal plastic joints can be extruded in the lining. Similar transverse joints are placed during finishing operations.

Efficient channel construction requires careful coordination of the successive operations. The trimming machine should be closely followed by the lining machine and by separate grooving and curing jumbos just behind the lining machine. If it is to proceed smoothly and effectively, this technique depends on an abundant supply of concrete with consistent properties. Such a supply is best furnished by a local premixed concrete batch plant and delivered in agitator trucks (Stevenson, 1999).

Precast concrete slab or block linings

Precast concrete has been used in lining works throughout the world for channels of all sizes, but the trend is declining in countries with expensive labour. Precast slabs are usually made 50-70mm thick and are factory cast, ideally using steel moulds and high-strength concrete. They are generally cast in sections that can easily be handled by two workers. They should be cured and stored for some months so that shrinkage occurs before use, minimising shrinkage in the channel (Stevenson, 1999). Joints provided on the block are sealed with mortar or bituminous mastic. Precast concrete slab linings in which all joints are sealed by a sealant such as bituminous mastic are flexible enough to absorb minor movements of the subgrade without damage. Precast concrete slabs are subject to deterioration on sharp curves, which should therefore be lined using another method.

Precast concrete linings of this type appear most promising for use by small maintenance crews in lining or repairing short sections of channel or laterals. No particular skill and very little equipment is required. However, the large amount of manual labour required in placing the blocks and sealing the joints makes this type of lining slow and too costly for extensive use (USBR, 1976).

Some water authorities have had success with the use of precast channel sections for smaller U-shaped channel profiles. These are made in a precasting yard and incorporate externally placed waterstops at each end (Stevenson, 1999).

The precast sections are placed as alternate sections on the channel foundations and the missing sections between them are then formed in-situ, using the precast sections as screed guides (Stevenson, 1999).

Concrete thickness Top button

There is no general rule for the thickness of concrete linings. For small canals and ditches in locations where severe frost action is likely to occur, unreinforced linings of about 40mm thickness have been satisfactory. In most countries with mild climates, concrete linings are 50-75mm thick for small channels and 75-100mm thick for larger channels. Under more severe climatic conditions or in channels with frequent changes in level and or unfavourable subgrades, thickness is increased and may exceed 150mm for large channels.

Concrete linings need to be fairly robust, yet as economical as possible. A lining thickness of 75-100mm seems to be the optimum for economic construction with good service life, depending on channel size (Stevenson, 1999).

Finishing and curing Top button

Proper curing greatly improves the durability, wear resistance and watertightness of concrete. A smooth hand-trowelled surface finish increases the carrying capacity of a channel and is justified where labour is inexpensive.

Curing involves keeping the concrete moist for at least a week after it has set, in order for it to achieve its maximum potential strength and impermeability. Good curing can be achieved by covering the work with hessian and then keeping the hessian wet (with soaker hoses or similar), by covering the concrete with plastic sheeting, or by spraying a film-forming curing membrane over the concrete surface (Stevenson, 1999).

Filling the channel with water would also work, but it is unlikely to be a practical option, since it would have to be emptied again for joint sealing. Early filling can also cause cracking in green concrete if there is any ‘give’ in the supporting soils (Stevenson, 1999).

Modern construction using slip forms, as described above, requires minimum or no finishing and curing of the concrete.

Joining Top button

Cracks result in concrete-lined channels due to contraction caused by drying, shrinking and temperature changes. To prevent uncontrolled cracking in the channel prism, cracks are confined to select locations by creating weakened planes or joints. Cracks are initiated at these locations, where they are easily sealed, and random cracking is minimised, which reduces leakage and consequent damage such as loss of foundation soils (Stevenson, 1999; Swihart and Rutenbeck, 2001).

Both transverse and longitudinal contraction joints are recommended in channels having a lined perimeter of 9m or more, particularly those that are unreinforced. Even smaller channels may require two-way crack control if the sub-base material warrants it. Transverse joints are normally spaced every 3-4.5m depending on the material, slab thickness and reinforcement (USBR, 1988).

Previously, contraction grooves were generally provided by cutting or forming grooves in the upper surface of the slab while the concrete was still plastic. In this method, transverse grooves are cut either by hand along a straightedge or by a mechanical knife or cutter impressed and vibrated into concrete. Longitudinal grooves are cut by stationary or revolving cutters attached to the rear of the slip form. Shrinkage cracks are then largely confined to the location of the grooves where the thickness of the lining had been reduced. Once cracks formed at these locations, the cracks are often left open, except where a high degree of watertightness is required. This policy was dictated partly for reasons of economy but also because asphalt-based sealers were virtually the only materials obtainable, and these were subject to rapid deterioration from weathering.

Extensive field and laboratory studies have resulted in the development of two basic systems for sealing contraction joints and one for sealing random cracks. For contraction joint sealing one of the following methods is recommended:
  • A polyvinyl chloride (PVC) strip waterstop inserted into fresh concrete.
  • An elastometric sealant extruded into joints in cured concrete.

For random crack sealing, the moulded-in-place cap strip formed from an elastometric joint sealant shows the most promise of long-term service (Swihart and Rutenbeck, 2001).

Figure 3 Cap seal for random cracks
Figure 3 Cap seal for random cracks
Source: Swihart and Rutenbeck, 2001.

Contraction joint-forming waterstops

Transverse contraction joints are provided in channel linings by inserting polyvinyl chloride (PVC) plastic contraction joint-forming waterstop. This consists of a plane-weakening vertical member added to a miniature waterstop, which is normally referred to as a PVC strip (as shown in the figure below). Extruded from PVC and inserted into the concrete during lining placement, this strip controls cracking effectively.

Figure 4 PVC strip waterstop
Figure 4 PVC strip waterstop
Source: Swihart and Rutenbeck, 2001.

Experience proves that the installation must be correctly made:

  • The top of the strip must not be more that 13mm below the concrete surface, or the contraction crack might not develop at the desired location.
  • The strip must not be tilted sharply, or the crack might lead to a sealing bulb, destroying the waterstop effect.
  • Installation must be made using plastic concrete and the strip is usually fed into the concrete and kept in tension to ensure proper depth and orientation.
  • Sufficient vibration is required to produce thorough consolidation of the concrete around the strip and to provide continuous contact between the concrete and all surfaces of the strip. With T-shaped water stops the vibration of the concrete around the water stop is critical (Stevenson, 1999).
The advantages of the PVC strip are as follows:
  • It forms a weakened plane in the lining, producing excellent crack control when properly installed.
  • It seals by waterstop action, so the seal is not dependent on bonding to the concrete.
  • It is buried, so weathering is virtually eliminated.
  • It is chemically inert and unlikely to be affected by extended water immersion.
  • It is a manufactured item receptive to high-quality control standards.
  • It withstands high hydrostatic pressures.
  • It enables lining and sealing a channel in a single operation.
  • It tends to unitise a channel lining by tying the individual slabs together, although this effect is reduced by stress relaxation

Elastomeric sealants

Elastomeric sealants are used in much the same manner as the previously used asphaltic mastics but with markedly better results. A groove is cut in the concrete while it is still plastic, similar to installing contraction grooves. Once the groove is formed, the elastomeric sealant is injected into the groove as presented in the figure below.

Figure 5 Channel contraction joint with elastomeric sealant
Figure 5 Channel contraction joint with elastomeric sealant
Source: Swihart and Rutenbeck, 2001.

A recent elastomeric sealant in use is a coal-tar extended polysulfide sealer that was modified from an airport runway sealer. It weathers well and resists hydrostatic pressure. It bonds well to concrete and remains bonded even after long periods of immersion in water. Being a two-component, quick curing material, it requires specialised equipment for successful application. Similar but slower setting polysulfide sealants are also available for hand mixing and placement on small jobs.

Performance Top button

A properly installed and maintained concrete lining provides satisfactory seepage control. The estimated effectiveness in seepage reduction is 90% with sealed joints, and the estimated lifespan is 40-60 years (Sinclair Knight Merz, 1998; Swihart and Haynes, 1999).

Concrete is more resistant to erosion than most other lining materials. It is therefore preferred for high water velocities. It is recommended that velocities in reinforced concrete channels should not exceed 2.5m/s (Swihart and Rutenbeck, 2001). Higher velocities are permissible in well-constructed reinforced concrete channels (USBR, 1988). In fact, properly designed and constructed reinforced concrete linings will withstand velocities of any magnitude considered feasible for channels (Swihart and Rutenbeck, 2001).

Linings of concrete eliminate most weed growth, with resulting improvement in flow characteristics and reduction in maintenance costs. Burrowing animals that cause breaks in other lining materials cannot penetrate concrete (Swihart and Rutenbeck, 2001).

Cost Top button

As the available field data is predominantly from the United States, all costs are converted from US to Australian dollars (1998). Costs are found to be similar to geomembranes because of the need to import material, although the side slopes can be less expensive. While properly constructed concrete linings have many advantages, concrete is sometimes not used due to its high cost (Swihart and Rutenbeck, 2001).

The capital costs for installing concrete lining was recently estimated to be approximately $47.5-52.5/m2 (Sinclair Knight Merz, 1998).

Failures Top button

Cement concrete linings have failed due to the following (USBR, 1976):

  • Adverse subgrade conditions, such as loss of support through piping action and bulging of expansive clays
  • Excessive hydrostatic pressures beneath the lining
  • Frost heaving
  • Surface damage from freezing and thawing
  • Poor quality concrete
  • Faulty design or construction methods
  • Combinations of these and similar factors.

Concrete is susceptible to damage from collapsible soils, expansive clays, alkali water, alternate freezing and thawing action, and from frost heave. Consideration of locality and proper design and construction can help reduce these problems (Swihart and Rutenbeck, 2001).

It is generally assumed that concrete linings are used to prevent seepage and that the subgrade is usually relatively free draining and above the groundwater level. Unfortunately this is not always the case, and serious damage has occurred to concrete channel linings on some projects due to uplift from high groundwater levels. Proper drainage is the only means of correcting the problem. However, it may be costly and complicated (Swihart and Rutenbeck, 2001).

If gross soil movement is a problem in the vicinity of the channel then concrete lining may not be the best solution, unless it is equipped with an external membrane to control leakage (Stevenson, 1999).
Concrete lining of channels has been used extensively for seepage control in irrigation schemes. However, it has been found to be cost-effective under Australian conditions only for high-value irrigated crops, and low-pressure pipelining is generally now preferred (Sinclair Knight Merz, 1998).

Repair

Top button

A comprehensive description of repairs to concrete-lined channels is provided in Stevenson, ‘Repair/replacement options for concrete lined irrigation channels’ (1999).

Related pages Top button

Hard surface lining techniques
Shotcrete
Grouted fabric mats
Soil-cement lining
Flumes and pipes
Tiles and bricks
Asphalt
   

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Page last reviewed on 25/6/04